A recent development in telecommunications technology has been the introduction of the asynchronous transfer mode (ATM) transmission technique. The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, e.g. voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a header indicating its destination. The service traffic is adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently.
Specification No EP-A1-0,225,714 describes a communications network comprising a number of nodes and in which short delay limits are met by creating composite packets carrying information for more than one call. A call is allocated one or more octets at a given location in packets having a given connection number. A description of voice transport in an ATM broad band network is given by W O Covington in "Communications Technology for the 1990's and Beyond, Dallas, Nov. 27-30, 1989, Volume 3, 27 November 1989, IEEE, pages 1921-1925.
A limiting factor in the introduction of ATM is the difficulty of interfacing new broad band ATM networks not only with existing narrow band networks, commonly referred to as legacy networks, but also with the newly emerging narrow band cellular and wireless networks. In an attempt to address the latter-mentioned part of this problem, ANSI committee T1S1.5 has recently issued a baseline document containing proposals for a new ATM Adaptation Layer (AAL) that encapsulates and transports short user packets (called AAL-SDUs) inside an ATM cell stream. The new AAL is intended to be applicable to both fixed and variable length short packets and has been named `Small Multiplexed ATM Adaptation Layer` (SMAAL). SMAAL is intended to support applications such as low bit rate-compressed voice, both with and without silence suppression, leading to both constant bit rate and variable bit rate user information. A consequence of these applications can be the unacceptably long voice packetisation delay that can occur in filling a complete ATM cell with the resulting user information from a single source. The purpose of SMAAL is to permit multiple short packets of user information from one or more users to be multiplexed inside a single ATM cell, thereby alleviating the packetisation delay problems. While this facilitates the transmission of short packets within the ATM network, it does not provide an effective means of interfacing with existing legacy networks, neither basic 64 kbit/s narrow band networks nor narrow band voice networks employing compression and silence suppression techniques.